In general, glycoconjugates are carbohydrate derivatives in which the carbohydrate part is linked to another chemical moiety by covalent linkage. Glycoconjugates like glycoproteins, glycopeptides, peptidoglycans, glycolipids, lipopolysaccharides play important roles in living systems, particularly in cell-cell or cell-matrix interactions.
Glycoproteins are proteins that contain sugar moiety/moieties, particularly oligosaccharide chain(s), covalently attached to the amino acid unit(s) of the polypeptide chain of a protein, preferably via the OH-group of an amino acid (serine, threonine, hydroxyproline) or the amide nitrogen of asparagine or glutamine (O- vs. N-glycosides). Glycoproteins are often parts of cell membranes. Glycopeptides are similar to glycoproteins in that the glycosyl moiety is attached to an amino acid, but that particular amino acid can be a single aglycon or the part of an oligopeptide.
Glycolipids are lipid (fatty acid) containing carbohydrate conjugates, generally classified in the groups of glyceroglycolipids, glycosphingolipids and glycosylphosphatidylinositols, with the role providing energy for the cell. Lipopolysaccharides, also lipid containing glycoconjugates, but having antennary-like polysaccharide chains, are components of the outer membrane of bacteria and involved in inducing immune response.
Preparing complex molecules such as glycoconjugates has always been a challenge for chemists and biologists. Structurally altered analogues of glycopeptides can be important targets in studying and understanding basic physiological processes taking place in cell systems. Glycoconjugates and analogues thereof can be synthesized by both chemical and enzymatic ways. In chemical synthesis, the carbohydrate part has been reacted with the aglycon under glycosylation condition, in which both the participating educts shall be present in protected form, that is all functional groups that are not involved in forming the desired linkage and can be affected under glycosylation conditions shall be masked. This chemical approach, in many cases, can suffer from low stereoselectivity with regard to the newly formed glycosidic bond, low overall yield due to the high number of protection/deprotection steps, the use of sophisticated and often expensive purification methodologies such as column chromatography, and sometimes the use of toxic reagents. Alternatively, enzyme mediated glycosylation reactions can require fewer or even just a few elementary steps and/or take place with remarkable stereo- and/or regioselectivity, but the narrow substrate specificity of the enzymes towards the donors and the acceptors often requires the use of different enzymes depending on the structure of the target, which makes it practically impossible to develop a general methodology.
The production of some oligosaccharides having lactose, galactose or GlcNAc at the reducing terminus, and optionally their allyl glycosides, has been reported by using genetically modified E. coli cells which have been able to internalize lactose, allyl galactoside, allyl lactoside or GlcNAc-OAll added as exogenous precursors to the fermentation broth (WO 01/04341, Fort et al. J. Chem. Soc., Chem. Comm. 2558 (2005), WO 2007/101862, EP-A-1911850, WO 2010/070104, US 2012/208181).